What would be the best finger positions to play this kind of lick? Should the 12th fret be barred to improve speed? I see them quite often in a lot of different songs but it's the speed of playing them whilst reducing noise on the fretboard that I find difficult.

yes I play that with a bar on the 12th and roll the finger so B & E are not ringing out together so after you pick 12 on E string you kind of roll the finger down so you can fret the B at 12 and E will stop ringing. Afer a while this technique will become second nature.-- john